Certain disclosures exist in the art regarding the formation of unsupported metal carbide compositions, including those which are catalystic, by the calcination of precursors for such catalysts. Some recent examples include: the following:
U.S. Pat. No. 3,976,749 to H. Wedemeyer teaches the formation of monocarbides of metals by forming a mixture of carbon with an oxalate of the metal and then decomposing the metal oxalate in the presence of an external source of carbon in a stream of hydrogen.
Japanese Patent Publication No. 54/107,500 also teaches the use of a source of extraneous carbon with an organic titanic ester in order to form titanium carbide fine powder ceramics.
M. A. Janney in U.S. Pat. No. 4,622,215 proposed that titanium carbide powder could be formed from a carbon precursor polymer and an organo-titanate as reagents. The titanium moieties in the resulting product are deemed to be substituents to the polymer chain(s) carrying the carbon moieties in the reaction product which is then converted into the desired ceramic after pyrolysis. The patent mentions that a gel is formed.
U.S. Pat. No. 4,948,762 to W. Krumbe et al. forms carbides by reacting metal-containing compounds with a reactive hydrocarbon-containing compound, which is polymerizable and which contains a carbon-hydroxy bond. J. D. Birchall et al., in U.S. Pat. Nos. 4,861,735 and 4,950,626 also describes the production of ceramic materials by reacting a compound containing a metallic or non-metallic element having at least two groups that are hydroxy-reactive with an organic compound containing at least two hydroxy groups. In U.S. Pat. No. 4,861,735 it is stated in the first Example that the carbide precursor is formed as a waxy solid.
U.S. Pat. No. 4,826,666 to R. M. Laine utilizes metal alkyls or carbonyls in the preparation of metal carbide precursors and illustrates only certain polymeric precursor structures at Col. 4, line 65 to Col. 5, line 32.
Metal carbides can also be formed by the pyrolysis of a composition containing the desired metal (such as derived from a metal alkoxide or metal halide) and a carboxylic acid residue (such as from a dicarboxylic acid). See U.S. Pat. No. 5,169,808.
More recently, in U.S. Ser. No. 156,670, filed Nov. 23, 1993, it has been proposed that catalytic metal carbides can be formed by the calcination of a guanidine compound, adduct, or derivative and a transition metal salt, such as a transition metal halide.
A variety of disclosures also exist in the art in regard to how to form a supported metal carbide catalyst.
The prior art describes the impregnation of a support with a water soluble source of the metal alone, followed by calcination to the metal oxide, with subsequent exposure of the oxide to carburizing gases, such as methane/hydrogen (See S. T. Oyama et al., Ind. Eng. Chem. Res., 27, 1639(1988)) or carbon monoxide (See P. N. Ross, Jr. et al., J. of Catalysis., 48, 42(1977)). Both carburization reactions necessitate the use of high temperatures on the order of about 900.degree. C. L. Leclercq et al., in U.S. Pat. No. 4,522,708, discusses several supported carbide systems, including work by Mitchell and co-workers in supporting molybdenum on active carbon and other work relating to Group VI metals on alumina (e.g., U.S. Pat. Nos. 4,325,843 and 4,326,992). In all these processes, uncontrollable gases lead to formation of deposited free carbon. This carbon is undesirable and have to be eliminated as described by the authors, by exposing the final catalyst to a stream of hydrogen gas.
D. Dubots in U.S. Pat. No. 5,196,389 describes a metallic carbide obtained by coating the support with two components: a suspension of a reducible compound of the metal and a solution of an organic resin forming compound followed by carburization at 700.degree. C.-1400.degree. C. A temperature of 1000.degree. C. was needed to carburize the reduced metal with the organic resin forming compound. During carburization, carbon may be deposited on the active metal carbide sites, rendering them useless. This deposited carbon leads to an artificially high surface area. This phenomena is described by Ledoux et al., J. of Catalysis, 134, 383(1992). The high temperature needed for carburization often leads to a highly crystalline material as described by its X-ray diffraction (XRD) pattern. Sharp peaks in the XRD indicate large crystallites, i.e., a small specific surface area. Yet, in one example, Ledoux claims surface areas of 147 and 168 m.sup.2.g.sup.l for Mo.sub.2 C and WC, respectively.
Flynn et al., in Inorganic Chemistry, 10, 2745(1971), describe a water insoluble guanidinium metatungstate of the formula (CN.sub.3 H.sub.6).sub.6 (H.sub.2 W.sub.12 O.sub.40).multidot.3H.sub.2 O, where the metal to guanidine ratio is 1:O.5. This insoluble compound will not dissolve in water to allow impregnation of porous materials for catalyst manufacture.
K. F. Jahr et al., in Chemische Berichte, 98, 3588-3599 (1965), describe a water soluble guanidinium tungstate (CN.sub.3 H.sub.6).sub.2 WO.sub.4 where the ratio of metal to guanidine is 1:2. This compound was prepared by the reaction of ethyl tungstate solution in ethanol with guanidine free base. This compound was also prepared by the instant inventor, using a different process as will be shown below, and was found not to give the desired catalytic tungsten carbide of this invention.